Use of Mobile Acoustic Tag Tracking Surveys to Further Evaluate Fixed-Array Survival Studies

While most acoustic telemetry studies rely entirely on data from fixed arrays, these data sets can be enhanced with data from mobile acoustic surveys. Fixed array data sets provide a passage history for each station, and complementary mobile surveys focus on the areas between stations. Two examples illustrating the benefits of mobile acoustic tracking are presented from salmonid studies in California’s Central Valley. Since 2009 mobile monitoring has been used during juvenile fall-run Chinook salmon (Oncorhynchus tshawytscha) survival studies on the San Joaquin River to improve interpretation of survival estimates by pinpointing tagged fish locations and potential sources of mortality.  A HTI datalogger and omni-directional hydrophone was used to record acoustic data every 0.25 river miles along the lower San Joaquin and Old rivers.  The datalogger was attached to a laptop computer and data were recorded and viewed in real-time using HTI’s AcousticTag program.  Although the term ‘mobile’ is used to describe the methodology, the boat was anchored in the center of the channel for a minimum of 5 minutes to ensure detection of tagged smolt that may be moving downstream, holding, or immobile (deceased).  In a separate study, mobile tracking of adult rainbow trout/steelhead (O. mykiss) was conducted in the Tuolumne River to determine the specific locations and micro habitat usage of individual fish. In this case, surveys consisted of actively searching for tagged fish from a raft outfitted with an HTI datalogger with GPS tracking capabilities. The timing, frequency and location of mobile surveys were dependent on environmental conditions, and detection data from fixed stations and previous mobile tracking. Environmental data were recorded for each fish detected: tag code, time of detection, location of detection (GPS coordinates), surface water temperature at the hydrophone, and macro habitat unit type.  Micro habitat usage (e.g. depth, substrate, association with features such as undercut bank, woody debris, large boulder, etc.) was also evaluated by using signal strength to more precisely estimate the location of each fish.  In some cases, after the general location of the tagged fish was determined, snorkel and underwater video techniques were used to document the fish location within the habitat unit, and its general behavior (spawning activity) and condition. Results of these two studies demonstrate that mobile telemetry is a valuable technique to complement fixed-station telemetry, interpret fish behavior, and confirm fish mortality between fixed stations.